Modular orthopedic implants are implants that comprise multiple components that are assembled at the time of surgery. In the case of a modular hip prosthesis, there may be separate stem, body, and neck segments. These orthopedic devices often use a self-locking taper to join the multiple components of the implant together. These tapers provide a strong frictional joint that resists the torsional loads seen in the implant. Due to the frictional nature of these locking tapers, large assembly loads are generally needed to completely and securely join the components. However, if the assembly loads are too large, detrimental stresses may be applied to the implant which may result in premature failure.
Prior assembly approaches commonly used impaction type devices, such as a hammer, to provide impulse loading to assemble the implants. While these approaches can be effective, they are also inconsistent. Factors such as hammer weight, the number of blows, and surgical technique can lead to assembly loads that are either too low (so that they result in ineffective locking) or too large (so that they impart undesirably large stresses in the parts), either of which may result in premature implant failure. In addition, in some cases where the patient's bone quality is poor, aggressive impactions can lead to fractures of the bone and poor surgical outcomes. Poor bone quality may also lead to under-assembled components because a large frictional interface is generally needed between the bone and the implant in order to produce enough resistance to fully assemble the components. Due to these concerns, alternate approaches for assembling these modular implants are needed.
Some current assembly approaches provide for an instrument that, through simple mechanical advantage, produces quasistatic opposing axial forces that assemble the implant without impact. Opposing axial force devices are commonly used in other applications such as hand riveting tools and manufacturing center tool spindles and tool collet holders. With these instruments it is important that the force being applied be measurable, accurate, and reproducible. Prior attempts at doing this, such as is disclosed in U.S. Pat. No. 6,238,435, typically use a design that measures the deflection of a beam subjected to an input load to calculate the axial tension generated. However, this approach generally lacks sufficient accuracy due to the change in mechanical advantage that occurs as the handles are brought together. More particularly, if the handles start far apart, the same force applied to the handle will produce a much smaller axial force than if the same force were applied with the handles close together. Other factors, such as where the surgeon applies the closing force along the length of the handle, will produce dramatically different axial forces as well.
The device as disclosed in U.S. Pat. No. 6,238,435, has a significant disadvantage when compared to an object of the present invention, when considering the indication of force. In U.S. Pat. No. 6,238,435, the force indicated is a function of the force applied to the handles. Because the force applied to the taper junction is a cosine function of the force applied to the handles, the surgeon is unable to detect or measure the force applied directly to the taper junction. It is an object of the present invention to give the surgeon a direct indication and measurement of the force applied to drawing the tapers together. As the spring element is compressed, the surgeon can see the amount of compression, and this compression is a direct and linear measurement of the amount of force applied directly to the taper junction.
The device as disclosed in U.S. Pat. No. 6,238,435, has a significant further disadvantage when compared to an object of the present invention, when considering the surgeon's perspective of the force applied to the handles, the angle between the handles, and the force applied to drawing the tapers together. In both devices, the force applied to the handles is very low prior to the engagement of the taper junction. However, once the taper junction is drawn together, using the device as disclosed in U.S. Pat. No. 6,238,435, the angle between the handles cannot be changed regardless of the force applied to the handles. In this “go/no go” situation, it is the surgeon's perspective that the handles move freely and then suddenly stop, the only additional motion detected by the surgeon is the bending deformation of the instrument itself.
It is an object of the present invention to give the surgeon greater visual and tactile feedback as a function of the applied drawing force once the taper is closed. Using the spring, or other element which has a change in length that is proportional to the applied force, once the taper is closed, the surgeon will continue to change the angle between the handles, which changes the length of the spring element, and thereby increases the force applied to the taper junction. This visual and tactile feedback allows the surgeon to finely adjust the amount of force applied to the taper junction.
No existing design directly measures the true axial force generated.
No existing design permits the generation of a pre-determined axial force.
It is, therefore, an object of the present invention to provide a device for assembling modular implant components that directly indicates the assembly force applied to the components.